1. Field of the Invention
The present invention relates to an image processing apparatus, an image pickup apparatus, a control method for the image processing apparatus, and a storage medium storing a control program therefor, which have functions for suppressing color fringing in a taken color image.
2. Description of the Related Art
Conventionally, in an image pickup apparatus capable of taking color images, chromatic aberration in an imaging optical system that makes an optical image of a subject be formed on an image pickup device may cause a color that does not originally exist to appear around a bright part of the image as color fringing. In visible light color imaging using an image pickup apparatus, color fringing easily occurs in a part away from green, which is the central wavelength for the imaging optical system, and an artifact in blue or red, or purple, which is a mixture thereof, appears as a fringe, which is called “color fringe” or “purple fringe” or the like.
Chromatic aberration in an imaging optical system in an image pickup apparatus can be optically suppressed by a combination of plural lenses having different dispersions. However, in recent years, as image pickup apparatuses (digital cameras) have been downsized, there is an increasing need for high resolution for an image sensor used for an image pickup device as well as downsizing of optical systems, which has caused difficulty in sufficiently suppressing chromatic aberration by the optical system alone. Therefore, there is a need for suppressing the aforementioned artifact by image processing.
Chromatic aberration is roughly divided into transverse chromatic aberration (magnification chromatic aberration) and longitudinal chromatic aberration (axial chromatic aberration). Transverse chromatic aberration is, as shown in FIG. 17, a phenomenon in which the image location shifts in a direction along the image plane according to the wavelength. Also, longitudinal chromatic aberration is, as shown in FIG. 18, is a phenomenon in which the image location shifts in a direction along the optical axis according to the wavelength. When a primary color-type digital imaging system is used, color fringing caused by transverse chromatic aberration can be corrected by geometric conversion in which different distortions are provided to respective color planes for R (red), G (green) and B (blue) (see, for example, U.S. Pat. No. 6,724,702B1).
Meanwhile, where an image is focused with reference to the plane for G (green), which exhibits the central wavelength of the visible light range, longitudinal chromatic aberration causes the image to be unclear (blurred) because the focal points on the planes for R (red) and B (blue), which reside in edges of the visible light range, cannot be brought to the subject. Color fringing caused by longitudinal chromatic aberration cannot be corrected by such geometric conversion as mentioned above.
A method for correcting color fringing using the characteristic of color fringing mainly occurring around a whiteout region (a region reaching a preset signal saturation level) has been proposed (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2007-133592). In the method, a saturated region of the G (green) plane is searched for, signals of pixels surrounding the saturated region are integrated to calculate the amount of correction, and color fringing is corrected. Also, methods in which the color saturation of a region expected to cause color fringing is lowered to diminish the appearance of color fringing have been proposed (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2001-145117).
Although in an image taken by an image pickup apparatus, color fringing occurs mainly around a whiteout region, color fringing that gives a feeling of discomfort to viewers occurs also in a region with no whiteout. For example, in an image taken of a scene of sunbeams streaming through the trees, distinct color fringing occurs at the boundary between the blue sky background with no whiteout and the tree branches.
However, a sufficient effect cannot be obtained by the processing of searching for a whiteout region and correcting color fringing in its surrounding region, like that disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2007-133592, alone. Meanwhile, the processing of lowering the color saturation of a region expected to cause color fringing, like that described in Japanese Laid-Open Patent Publication (Kokai) No. 2001-145117, provides an effect of eliminating the color in color fringing to reduce unnaturalness, but may lower the color saturation of a region with no color fringing because the processing also affects the original colors of the subject and a processing-target region is determined regardless of whether or not the region actually has color fringing.